1. Field of the Invention
This invention relates to a method for producing single crystals by Czochralski method. More particularly, the invention relates to a method for producing single crystals free of defects such as dislocation or facet.
2. Brief Description of the Prior Art
When it is intended to grow a single crystal of good quality by the Czochralski method, a seed crystal is contacted with a melt and then the seed crystal is pulled up while rotating it to form a part of a relatively small diameter. For example, when a crystal of Si is grown, the length of this small diameter part (hereinafter referred to as a "slender part") is generally 7 to 10 times the diameter of the seed crystal. After formation of its slender part, the diameter of the crystal is gradually increased and then the diameter is made constant to grow a single crystal of a large diameter free of a dislocation defect as a body portion. Either dislocation present in the seed crystal or dislocation introduced at the time of seeding can be taken out in the surface and removed therefrom by forming the above-mentioned slender part.
In conventional methods for growing single crystals according to the Czochralski method, the velocity of rotation of the crystal is not changed during the growing step, and the velocity is maintained at a constant level so that a facet which is a defect owing to the habit of the crystal is not formed. Since a facet is formed because of the presence of a habit plane having a certain angle to the crystal pulling-up axis, removal of the facet can be accomplished by leveling the convex form of the solid-liquid interface on the melt side and keeping the interface substantially vertically to the pulling-up axis. The form of the liquid-solid interface varies depending on the velocity of rotation of the crystal, and at a lower rotation velocity the form of the interface on the melt side is more convex. Accordingly, the above condition is satisfied by heightening the rotation velocity to such an extent that the interface form is not too concave and thus the facet is removed.
In removing a dislocation caused by growth of the slender part, no particular problem is brought about on the form of the solid-liquid interface in conventional methods, and the object of removal of said dislocation can easily be attained. More specifically, the crystal plane in which a dislocation is formed is usually at (111), and therefore, all the dislocation is removed to the surface by choosing the pulling-up axis suitably.
However, as a result of our research efforts, it has been found that it is true in all crystals that dislocation defects of the slender part can be removed regardless of the form of the solid-liquid interface. For instance, in growing a single crystal of gadolinium gallium garnet (Gd.sub.3 Ga.sub.5 O.sub.12 which will be referred to as "GGG" hereinafter) to be used as a substrate of a bubble device now highlighted as a memory element for an electronic computor, when the conventional techniques of the Czochralski method are directly applied, it is very difficult to obtain a single crystal of good quality. More specifically, if a slender part is formed at such a crystal rotation velocity as will form no facet, removal of the dislocation is very difficult, and if it is intended to remove the dislocation by all means under the above rotation velocity condition, it is necessary to increase the length of the slender part extremely, with the result that it takes a long time to complete the preparation of the intended single crystal and the amount of Ga evaporated during formation of the slender part is so large that there is brought about such a great defect that the composition of the resulting single crystal is greatly deviated from the intended composition.
If a facet is generated, distortion occurs at this portion, and further, if other element is doped, segregation is caused to occur. Accordingly, in order to obtain a single crystal of good quality, it is necessary to avoid not only dislocation but also facet defects. In case GGG or the like is used as a substrate of a bubble device, defects such as dislocation and such as facet result in trapping of a bubble domain and hence, the presence of such defects is not permissible.